Webster’s New World Medical Dictionary provides a simple definition of a biomarker as “a biologic feature that can be used to measure the presence or progress of disease or the effects of treatment.” In the research literature, the term “candidate biomarker” is often loosely applied to any biological feature associated with a disorder. The search for biomarkers for psychiatric disorders has a long history, with earlier studies investigating molecular markers, like platelet imipramine binding or cerebrospinal 5-hydroxyindoleacetic acid (5-HIAA) in people with depression, or behavioural markers, such as smooth pursuit eye movements in people with schizophrenia. More recent searches continue to look for blood biomarkers (e.g., brain-derived neurotrophic factor and cytokines in people with schizophrenia or depression) and behavioural biomarkers (e.g., fear extinction in people with anxiety), and are progressing on to more high-tech searches, such as the use of proteomics to define signatures of proteins altered in blood or the use of support vector machine analysis to define informative neuroimaging patterns or panels of immunoassays altered in specific disorders.